The present invention relates to turbines and particularly to dovetail joints between the wheel of a steam turbine rotor and steam turbine buckets.
Dovetail attachment techniques between turbine buckets and turbine rotor wheels for steam turbines are well known in the art. Conventional tangential entry dovetails on the latter stages of low-pressure rotors operating in a contaminated steam environment have been found to be conducive to stress corrosion cracking (SCC). SCC is accelerated by the stress levels that are present in the hook fillet regions of typical dovetail configurations. Normally, these stresses are acceptable but with contaminated steam, cracks can initiate and, if left undetected, may grow to a depth that will cause failure of the wheel hooks. In extreme cases, all the hooks will fail and buckets will fly loose from the rotor. Long experience with bucket-to-wheel dovetail joints has indicated that the wheel hooks crack but that the bucket hooks do not crack. This is apparently because the NiCrMoV and similar low-alloy steels used for low-pressure rotors are much less resistant to SCC than are the 12Cr steels used for buckets. The steels for the wheels give the optimum combination of properties available for overall low-pressure rotor design considerations. Thus, an effective means of avoiding SCC in the typical low-pressure steam environment is to reduce the stresses in the wheel dovetail to acceptable levels. If the maximum stress in components operating in a corrosive environment is below the yield strength of the material, the resistance to SCC is greatly improved.
Bucket and wheel dovetail designs for steam turbine rotors have been described and illustrated in U.S. Pat. Nos. 5,474,423, 5,494,408; and 5,531,569, of common assignee. In U.S. Pat. No. 5,474,423 the dovetail joint design provides four hooks on the rotor wheel which decrease in thickness from the radially-outermost hooks to the innermost hooks. Additionally, fillets are provided between neck portions of the rotor wheel dovetails and bottom surfaces of the overlying hooks, with multiple radii, i.e., compound fillets, in order to decrease the stress concentrations with increased radii of the fillets. Additional features of that prior design include a flat surface along the radially-outermost surface of the hook and in combination with various forms of compound fillets. In U.S. Pat. No. 5,494,408 different fillet radii are provided between the hooks. In U.S. Pat. No. 5,531,569, compound fillet radii are disclosed.
In accordance with the present invention, a rotor wheel and bucket dovetail joint design is provided which minimizes concentrated stresses caused by the centrifugal force of the buckets in the wheel hook fillets, and permits larger hook fillet radii which further reduces stress concentration. In accordance with a principal aspect of the present invention, the rotor wheel contact surfaces, i.e., the generally radially-inwardly facing surfaces along the undersides of the wheel hooks are provided with identical slant surface angles for each hook of the dovetail at different radii along the dovetail. It will be appreciated that the rotor rotation causes the buckets to develop centrifugal forces which are imposed on the dovetail through the contact surfaces along the undersides of the wheel hooks. These forces give rise to stresses in the dovetail with peak stresses in the fillet regions of the hooks. The slant surfaces reduce the stress concentration for a given fillet radius and permit larger hook fillet radii that further reduce the stress concentration.
More particularly, the crush surfaces for traditional tangential entry dovetails are on an axial-circumferential plane with a fillet used as a transition between the crush surface and the neck surface at the various locations along the dovetail. These two surfaces are 90xc2x0 apart in conventional tangential entry dovetails. In U.S. Pat. No. 6,142,737, of common assignee, the two surfaces are greater than 90xc2x0 apart but vary from hook to hook. In the present invention, these crush surfaces are rotated such that the transition angles between the crush surfaces, i.e., slant surfaces, and the neck surfaces (in a radial plane) are greater than 90xc2x0 and are the same at each hook radii. The angles of rotation are called slant angles. Concentrated stresses result when load paths are forced to change direction. With the slanted crush surfaces hereof, the change in direction from 90xc2x0 to larger angles is less severe and the stress concentration is therefore lower. The slant crush surface also permits a larger fillet radius in the same transition distance as compared to the conventional 90xc2x0 transition, with a resulting larger fillet radius and lower concentrated stress. It will also be appreciated that a slanted crush surface causes a component of force in the axial direction which gives rise to bending of the bucket leg and an axial load on the tang of the wheel dovetail. To minimize this effect, the slant angle is constant from hook to hook, i.e., the same slant angle is provided at each hook radii. Because the slant angles of the crush surfaces are increased in angle from 90xc2x0, the fillet radii are also increased and stress concentrations thereby reduced.
In a further aspect, it will be appreciated that hook thickness and length control the load sharing between hooks, as well as the bending and shear stresses on hooks. Consequently, the hook thickness is varied to achieve uniform and minimum concentrated stresses, i.e., hook thickness increases with decreasing radial height.
The invention as described herein relates to both three hook and four-hook dovetail designs. The invention is also useful with other dovetails with any number of hooks. Additionally, the invention is not limited to rotors susceptible to SCC and the benefits and advantages hereof can be realized for other stress-causing conditions which initiate cracking in dovetail hooks such as dovetail cracking in high-temperature regions when creep is the failure mode rather than SCC.
In a preferred embodiment according to the present invention, there is provided a dovetail joint between a rotor wheel and a bucket rotatable about an axis, comprising a male dovetail component on the rotor wheel and a female dovetail component on the bucket, the male dovetail component receiving the female dovetail component in a direction tangential to the rotor wheel, the male dovetail component including a plurality of circumferentially extending hooks lying on opposite sides of a plane normal to the axis and bisecting the male dovetail component, each hook having a generally radially inwardly-facing surface, the surfaces of at least a pair of hooks on each of the opposite sides of the plane defining angles extending away from the plane and toward and away from the axis, the angles of the surfaces of each pair of hooks on each of the opposite sides of the plane being equal to one another.
In a further preferred embodiment according to the present invention, there is provided for use in a steam turbine dovetail joint that has a constant angle contact surface between a rotor wheel and a bucket both rotatable about an axis, the combination of the rotor wheel and bucket wherein the rotor wheel includes a male dovetail component for receiving the female dovetail component in a direction tangential to the rotor wheel, said male dovetail component including a plurality of circumferentially extending hooks lying on opposite sides of a plane normal to the axis and bisecting the male dovetail component, each hook having a generally radially inwardly-facing surface with the surfaces of each of the hooks on opposite sides of the plane defining angles extending away from the plane and toward and away from the axis, the angle of each surface being equal to the angle of every other surface, a female dovetail component on each bucket including a plurality of circumferentially extending hooks generally complementary to the male dovetail hooks and having radially outwardly directed angled surfaces generally complementary to the angled surfaces of the male dovetail component, the angles of the surfaces of the female dovetail component being equal to one another.